docs: def default(thumbnailer, prepared_options, source_filename,
              thumbnail_extension, **kwargs):
      """
      Easy-thumbnails' default name processor.
  
      For example: ``source.jpg.100x100_q80_crop_upscale.jpg``
      """
      filename_parts = [source_filename]
      if ('%(opts)s' in thumbnailer.thumbnail_basedir or
              '%(opts)s' in thumbnailer.thumbnail_subdir):
          if thumbnail_extension != os.path.splitext(source_filename)[1][1:]:
              filename_parts.append(thumbnail_extension)
      else:
          filename_parts += ['_'.join(prepared_options), thumbnail_extension]
      return '.'.join(filename_parts)